Electric measuring instrument



" '8. w.- sIT. CLAIR ELECTRIC umsunme 'iusrmm Sept. 23, 1930.

Filed s epf. 13,1929

Inventor. Byr-on W. St. Clair; 13

His A't'towney Patented Sept. 23,. 1.930

' UNITED STATES.

PATENT- OFFICE BYRON W. ST. CLAIR, OF LYNN, MASSACHUSETTS; ASSIGNOB TOGENERAL ELECTRIC COMPANY, A. CORPORATION NEW YORK ELECTRIC MEASURINGINSTRUMENT Application fl1'ed8eptember 13, 1929. Serial No. 392,457.

My invention relates to electric measuring instruments and its principalobject is to compensate alternating current instruments for variationsin frequency. Another object is to provide a magnetically damped electric measuring instrument of improved design.

The novel features of the invention will be explained as applied to analternating current instrument having a stationary magnetic circuitprovided with an exciting winding and a magnetic vane armature. Such aninstrument is suitable for use as an alternating current ammeter orvoltmeter and the accompanying drawing illustrates the essentiahfeaturesof an alternating current ammeter of this type to which I have appliedmy invention with satisfactory results.

Instruments of this character have been subject to frequency errors whenemployed on varying frequency circuits due to the iron losses,particularly to eddy current losses in the magnetic circuit. Theselosses increase with an increase in frequency and if uncom pensated,reduce the instrument torque with an increase in frequency and produce acorresponding error in the measurement. In a voltmeter the reactance ofthe'winding varies with frequency and produces a frequency error thatmaybe compensated by means of my invention. q

Eddy current losses maybe reduced by suitably laminating and. slottingthe magnetic circuit and such expedients' are preferably included in myinstrument but where the instrument is to be used with frequencyvariations, from 60 to 600 cyclesfor example, some frequencycompensating feature such as the one herein described is necessary tobring the frequency error of the instrument within the allowable limit.In accordance with my invention I shunt a portion of-the instrument,flux around the magnetic vane armature and vary the shunted fluxinversely with the frequency to the extent necessary to accom lish theresults desired. Other features. 0 the I invention will hereinafter beexplained and such features as are considered to be new and patentablewill be pointed out in the claims.

appended hereto.

with a magnetic damper to steady the 'indications.

The stationary magnetic circuit is in the general shape of a U andcomprises a laminated base or yoke portion 10, laminated pole pieceportions 11, together with core pieces 12 joining the pole pieces 11tothe base 10. The core pieces may also be laminated. For example, theymay be formed by rolling up a sheet of magnetic material in a closelywound spiral. I have also found it helpful to slot the laminated polepieces as shown at 22 .where they surround the core pieces 12. Theenergizing winding 13 is wound on thecore pieces in two sectionsconnected in series as shown to produce a flux through the magneticcircuit and across the air gap between the pole pieces 11. In parallelwith the pole pieces 11 and the armature air gap I provide a branch orshunt flux path through auxiliary pole pieces 14 integral with polepieces 11.- There is a comparatively narrow air gap 15 between theseauxiliary pole pieces and surrounding the auxiliary pole pieces 14 areshort circuited coils 16 formed of a single turn of a heavy copperconductor. A non-magnetic yoke 17 is provided between the pole piecesfor supporting the upper bearing of the instrument and the armaturerestoring spring. This yoke is broken away and the parts just mentionedare omitted from the drawing in order that the fre quency compensatingshunt may. be more clearly seen. a

The movable vane armature 18 is a crescent-shaped sector 'ofmagnetic'.material and is mounted on the shaft 19 between the circularshaped faces of pole pieces 11. The instrument-pointer and scale arerepresented at 20 and 21 respectively. The armature 18 is shown in aposition correspondin to a low instrument deflection and it will eunderincreased the armature 18 will be moved in a clockwise directionagainst the torque of the control spring until the enlarged por- .stoodthat as the excitation of winding 13 is p tion of the armature sector isopposite the narrow portion of the air gap between the poles 11 andthat'the latter position will correspond to maximum instrumentdeflection.

The material of which the magnetic circuit and armature is formed andthe extent to which the circuit is laminated have a bearing upon thehysteresis and eddy current losses and the fre uency error of theinstrument. Where it is own that the instrument will be used only oncircuits having a comparatively narrow range of frequency variation aproper selection of the magnetic materials used and their properlamination will sufiice to reduce the frequency errors to a negligiblevalue.- Some instruments are employed on different frequency circuits oron circuits over which the frequency is varied several hundred per centand in such cases it is necessary to employ frequency compensation andwhen frequency compensation is employed the choice of magnetic materialsand the degree of lamination are not so im ortant since I have foundthat by means of my frequency compensating feature I may obtain anydegree of compensation desired.

It will thus be evident that the characteristics of the magneticmaterials, the degree of lamination, and the necessity-for compensationand the extent of such compensation will depend to some extent upon theuse for which the instrument is intended. The cost of the instrumentshould alsobe considered in selecting the design.

For the magnetic vane armature I have found that an annealed alloy ofiron and nickel, the nickelcontent being in the neighborhood of'78%, issatisfactory. The material to which I refer is prepared substantially inaccordance with the teachings of Elmen Patent 1,586,883, June, 1926,although the commercial material which I have used may vary somewhatfromthat described in the patent. This material has high permeability andlow iron losses. The poles 1] and the yoke plate 10 may be made oflaminated silicon steel and the core pieces upon which the coils arewound may be made of annealed Norway iron. My experiments indicate thatsuch an instrument without compensation will have a frequency error inthe neighborhood of -1% between 30 and 120 cycles, and 8% between 30 and600 cycles. By compensation these errors may be changed to about i5 and+1% re spectively. I do not intend to limit my invention to thisparticular design but give'it as one practical illustrationo theapplication-of my invention in a low cost instrument.

The frequency compensation of the instrument is accomplished by varyingthe relative magnitude of the total flux allowed to cross the armatureair gap. A certain amount of flux is shunted around the armature air gapthrough the poles 14. Due to the reactance increase in frequency. a

The instrument is preferably provided with a magnetic damper and inorder to combine this in a compact instrument without interfering withthe meter accuracy I have found that certain precautions are necessary.The magnetic damper comprises a stationary permanent magnet 23 with itspole pieces 24 and 25 opposite a magnetic keeper 26 and a sectorshapedconductor 27 carried by the instrument shaft 19 between the pole pieces24, 25 and the magnetic member 26. The shaft 19 extends through the yokeplate 10 at right angles thereto parallel with the leg or core ortionsof the instrument magnetic circuit.

t willbe noted that the yoke 10 is slotted at 28 so that the instrumentflux path therethrough is to the rear of the shaft as pictured and quiteremote from the pole pieces of the permanent magnet. The pole pieces 12where they join at right angles with the yoke 10 are quite remote fromthe permanent magnet pole pieces and what is more important they aremechanically and magnetically symmetricaL ly placed with respect to thepole pieces of the permanent magnet. The permanent magnet is evenlyspaced away from the yoke 10 by a double function. It carries theinstrument flux and serves as a magnetic screen between the permanentmagnet and instrument.

The importance of the symmetrical arstrument pointer from zero wereexperienced after removal of the instrument current.

This'was caused evidently by leakage flux from the permanent magnetcombining with nuts 29 at front and back. The yoke 10 serves theresidual flux in the magneticcircuit of the instrument. The directionand magnitude of the residual instrument flux of course depends uponthe'flux direction and magnitude when the instrument circuit isinterrupted and it may be in either direction in varying amounts or itmay even be zero. The resid: ual flux alone is insufiicient to cause anymaterial departure of the instrument pointer an uncertain andtroublesome zero indication from its true zero, but with an unsymmetri;7

was experienced. After this symmetrical arrangement illustrated in thepatent drawing was used this trouble was eliminated.

The permanent magnet' flux crosses from one pole piece 24 to the keeper26, then to the pole piece 25 and then around the yoke of the permanentmagnet which. is largely hidden in the illustration'by the instrumentyoke 10. This damping flux thus cuts the conductor 27 which is suspendedin the air gap between the poles of the permanent magnet and keeper 26and if the conductor is moving eddy currents are produced which tend todamp the movement.

In accordance with the provisions of the patent statutes I havedescribed the principle of operation of my invention, together with theapparatus which I now consider to represent the best embodiment thereof,but I desire to have it understood that the apparatus shown anddescribed is only illustrative and v that the invention may be carriedout by other quency comprising means for varying the proportion of theflux of saidelectromagnet which influences the armature.

2. An alternating current electric measuring instrument comprising astationary magnetic circuit provided with pole pieces spaced apart toform an air gap, a winding on said circuit for producing a flux acrosssaid air gap, a magnetic vane armature actuated in proportion to the airgap flux, a flux path in shunt to said armature air gap, and a closedcircuited conductor linked with said shunt flux path.

3. An alternating current measuring instrument comprising anelectromagnet provided with pole pieces spaced apart to-form an air gap,a movable magnetic vane armature member in said air gap, auxiliary polepieces spaced apart to form an auxiliary air gapin shunt relation tosaid first mentioned pole pieces and air gap, and short circuited coilssurrounding said auxiliary pole pieces.

4;. An alternating current measuring instrument comprising a magneticcircuit provided with two sets of pole pieces spaced apart to form airgaps in parallel relation with each other in said magnetic circuit, amovable magnetic vane armature member in one of said air gaps, shortcircuiting coils on the pole pieces adjacent the other air gap, and analternating current energizing winding for said magnetic circuit.

5. An alternating current electric measuring instrument having amagnetic circuit provided with an air gap, a movable magnetic vanearmature member in the air gap, a winding on the magnetic circuit forproducin atorque flux in said armature, and means %or compensating saidinstrument for variations in frequency comprising means for diverting aportion of the flux of said magnet around the armature air gap, andmeans for varying the proportion of the flux thus diverted posite sideof said base portion to the armature, and a permanent magnet forproducing a damping flux in said damping vane, the

poles of said permanent magnet being symmetrically disposed with respectto the leg portions of said U-shaped magnetic circuit.

7. An electric measuring instrument having a U-shaped electromagnetprovided with an air gap, a movable armature member in said air gap, amagnetic damper for saidmovable armature having a stationary permanentmagnet, the yoke portion-of said U- shaped electromagnet lying betweenthe permanent magnet and the armature and the pole pieces of thepermanent magnet being symmetrically disposed with respect to the legportions of the electromagnet.

8. An electric measuring instrument having a U-shaped electromagnet,pole pieces spaced apart between the open ends of the U- shapedelectromagnet toform an air gap, a movable armature in said air gap, ashaft upon which said armature is mounted extending parallel with thelegs of the U and be- )yond the yoke portion thereof, a vane ofconducting material on the extended portlon of said shaft, a permanentmagnet spaced apart from the yoke portion of the magnetic c1rcuitadjacent the said vane for producing a damping flux therein, said yokepbrtion forming a magnetic screen between the permanent magnet andtheremainder of the magnetic circuit of the instrument. x

9. An electric measuring instrument having an electromagnet formed by alaminated plate yoke portion, core pieces extending at right anglestherefrom in one dlrection, an

pole pieces spaced apart to form an air gap between the outer ends ofsaid core pieces, a movable armature in said air gap, a shaft on whichsaid armature is mounted extending through the yoke portion of saidelectromagnet, a dam ing vane on the extended portion ofsaid shaft, apermanent magnet energizing winding on said core pieces, and

spaced apart from said yolk portion on t e side opposite said corepieces for producing a damping flux in the vane, the pole pieces of thepermanent magnet being sym- 5 metrically placed with respect to the corepieces and the yoke forming a magnetic screen between the permanentmagnet and measuring instrument.

10. An electric measuring instrument having a U-shaped magnetic circuit,a movable armature between the open ends of said U- shaped circuit, anenergizing winding on the legs of the U-shaped circuit, the base or yokeportion of the magnetic circuit comprising a plate of inagneticmaterial, and a permanent .magnet damper for said instrument on theopposite side of said plate from said armature, the permanent magnet ofwhich is symmetrically disposed with respect to the leg portions of theU-shaped magnetic circuit. I

11. An alternating current measuring instrument having a U-shapedelectromagnet, a

magnetic vane armature between the open ends of the U-shapedelectromagnet, an energizing winding on the legs of the U-shapedelectromagnet, a permanent magnet adjacent the base or yoke portion ofthe U-shaped electromagnet and on the side 0 posite from the magneticvane armature, sai d permanent 3o magnet forming a part of a magneticdamper for the instrument, the pole pieces of the permanent magnet beingsymmetrically placed with respect to the leg portions of theelectromagnet. 12. An electric measuring instrument comprising amagnetic circuit provided with an 'air gap, the magnetic circuit beingof a' U shape with the air gap between the open ends of the U, the baseor yoke ortion of 40 the circuit comprising a laminate plate, an

energizing winding on the leg portions of the U-shaped magnetic circuit,a movable armature between the pole pieces, a shaft on which saidarmature is mounted extending through the yoke plate at right anglesthereto, a

damping vane carried on the extended portion of the shaft, a permanentma net secured to the yolk plane and space therefrom in a parallel planeon the side away so from the instrument, said permanent m et cooperatingwith the damping vane to orm a magnetic damper for the instrument, the

pole sieces of the permanent magnet being space awa from the flux pathof the instrumentt roughthe yoke but the yoke having extensions towardthe permanent magnet poles and formin a flux screen between theinstrument an damper, the pole 4 pieces of the permanent magnet beingsymmetrically s aced from the leg portions of "the U-shape magneticcircuit of the instrument. I

In witness whereoi I have hereunto set my hand this 10th day of Setember, 1929. BYRON W. T. CLAIR.

